A shoe press is a pressing (dehydrating) method in which an object to be pressed (wet paper web) is placed on the outer periphery of a press belt, and a surface pressure is applied to the object between a press roll positioned outside the periphery of the press belt and serving as external pressing means and a pressure shoe positioned inside the periphery of the press belt and serving as internal pressing means, through the press belt. While a roll press for performing pressing with two rolls applies a linear pressure to an object to be pressed, the shoe press can apply a surface pressure to the object to be pressed by using the pressure shoe having a predetermined width in a travel direction. Thus, performing a dehydrating press with the shoe press is advantageous in that a nip width can be increased and dehydrating efficiency can be improved.
In order to make the shoe press compact, a shoe press roll in which a pressure shoe serving as internal pressing means is covered with a flexible cylindrical press belt (press jacket) and assembled into a roll shape has been widely used as disclosed in, e.g., Japanese Patent Publication No. S61-179359 of unexamined applications.
General required characteristics for the press belt include strength, abrasion resistance, flexibility, and impermeability to water, oil, gas, and the like. Polyurethane, which is obtained by a reaction between a urethane prepolymer and a curing agent, has been commonly used for the press belt as a material having these characteristics.
In a papermaking technique, it has been known to form a multiplicity of drain grooves, extending along a belt travel direction, in the outer surface of the press belt in order to drain water squeezed from a pressed wet paper web.
FIG. 1 is a cross-sectional view showing a conventional typical press belt having drain grooves. A press belt 80 shown in the figure includes a multiplicity of drain grooves 81 extending along a belt travel direction, and a multiplicity of lands each positioned between adjacent drain grooves and extending along the belt travel direction. The size of the press belt 80 is generally as follows. The circumference is about 1 to 30 m, the width is about 2 to 15 m, and the thickness is about 2 to 10 mm.
FIG. 2 shows a state in which a wet paper web 84 to be pressed and a felt 83 are interposed between the press belt 80 and a press roll 85. This state is a state before pressing. An upper surface of each land 82 is flat, and this flat upper surface is in surface contact with the felt 83.
When a pressing operation is performed from the state shown in FIG. 2, an upper part of each land 82 is pressed downward and swells sideways as shown in FIG. 3. This reduces the size of the opening of each drain groove 81, thereby reducing the water squeezing performance (draining performance). If a permanent set occurs due to repeated pressure deformation, the drain grooves 81 become wide in the bottom and narrow in the opening or in the middle part, making it more difficult to drain the water entering the drain grooves 81. This results in so-called “water recirculation,” i.e., a phenomenon in which the press belt 80 containing water comes in contact with a wet paper web again. When such s phenomenon occurs, water cannot be squeezed from a wet paper web, and the paper is further moistened (remoistening).
The above problem results from the fact the lands are compressed and swell sideways by pressing, and the drain grooves are deformed. There is another problem. This problem will be described below with reference to FIG. 4.
FIG. 4 shows a state in which the felt 83 and the wet paper web 84 are placed on the press belt 80. As shown by arrows in the figure, water contained in the wet paper web 84 and the felt 83 located in the most region is squeezed into the drain grooves 81. However, due to a long distance from a region A located in the middle part of each land 82 to an adjacent drain groove 81, water is not sufficiently squeezed from the wet paper web 84 and the felt 83 located in the regions A. This causes non-uniformity in moisture distribution and fiber orientation in the wet paper web 84, which may adversely affect the paper quality. It is possible to increase the number of drain grooves 81 in order to sufficiently squeeze water in the regions A as well. In this case, however, the surface area of the lands 82 becomes too small, and the load is concentrated on the small area, whereby the pressure deformation of the lands 82 shown in FIG. 3 becomes more significant. As a result, not only the water squeezing property is not improved, but also the lands 82 themselves tend to break due to insufficient strength.